When it is desired to have access from the outside in a volume of an homogeneous magnetic field, it is provided to make a system comprising a minimum of four coils, the arrangement of the windings of which is assumed to be substantially on an imaginary approximative sphere. There is thus formed, in the center of the volume limited by the four coils, a magnetic field which is substantially homogeneous. A construction is therefore made which is very interesting both with respect to the number of necessary amperes/turns and easiness for adjusting the coils.
Theorically inscribing the coils on a periphery of an imaginary approximative sphere seems to be simple but for obtaining this result on a great volume, it is extremely difficult to place the coils and then to maintain the coils, since the coils have a tendancy to expand because of the passage of the current, and react the one on the other. Thus, the coils have a tendency to move the one with respect to the other, which is disadvantageous. Furthermore, it is necessary to take into account the environment which may contain interference magnetic fields, magnetic fields created by the iron body of a reinforced concrete building, a passage of one or a plurality of power supply cables, a more or less far positioning of various magnetic masses.